Radiation-crosslinked polyolefin compositions

[0001] The present invention relates to radiation cross-linked polymer compositions as well as coatings and insulating materials containing these compositions.

BACKGROUND OF THE INVENTION [0002] Polypropylenes are ideally suited for the production of coatings and insulation materials intended for use at operating temperatures above which other polyolefins such as, for example, polyethylene, which has lower softening and melting temperatures, cannot be used. Polyethylenes have a maximum melting point, as measured by differential scanning calorimetry (DSC) of about 135 ° C, while polypropylenes can have melting points as high as 175 ° C. As such, polypropylenes can withstand higher operating temperatures without being permanently damaged or deformed.

[0003] Other attractive properties of polypropylenes include their high rigidity, abrasion resistance, impact resistance, toughness, low cost and relatively low density. Applications for polypropylene-based coatings and insulation materials include insulation for electric wires and cables, heat-shrink, corrosion-proof insulation sheaths for connectors for high-temperature transmission piping, heat-shrinkable tubing or profiles for electrical insulation and mechanical protection, or other applications requiring greater resistance to dynamic load and stiffness, than provided by polyethylene-based systems.

[0004] In order to maximize heat resistance and physical properties, it is necessary to give the material certain thermosetting properties. This is accomplished by crosslinking the polymer to some required degree. Crosslinking gives the material resistance to melting and plastic deformation when it is heated to a temperature close to or higher than the melting point of the crystalline substance of the polymer component of the composition with the highest melting point. This property is necessary for the production of high-temperature insulation materials and heat-shrinkable products. In the latter case, crosslinking gives the material controlled shrinkage properties and melting resistance when it is heated to the temperature necessary to cause shrinkage of the material.

[0005] Various examples of cross-linked, predominantly polypropylene-based polymer compositions are known in the art. In the patent description Ser. No. 6,569,915, heat-shrinkable products are described which contain a radiation cross-linked composition predominantly based on polypropylene and also containing ethylene propylene elastomer. Patent descriptions Ser. US 6,455,637, US 6,465,547 and US 6,794,453, and International Patent Application WO 2002/32983 relate to products including moisture crosslinked, predominantly polypropylene based compositions that also contain some amounts of silane grafted polyolefin. All compositions predominantly based on polypropylene, which have been disclosed and claimed in these patents, are characterized by high rigidity, resistance to dynamic load and resistance to heat deformation, which makes them suitable for use in demanding applications such as protective insulating sheaths for high temperature transmission pipelines at which temperature

Operating temperatures typically range from 110 to 130 ° C, or higher.

[0006] However, there are situations in which it is desirable to provide compositions and articles having similar stiffness, dynamic load resistance and mechanical indivisibility, which are similar to the properties of crosslinked polypropylene-based compositions but with moderate high temperature fitness, which is determined by those mechanical properties that depend on the performance of the constituent polymers above their softening point. However, these compositions should provide similar long-term thermostability with respect to useful life at the intended operating temperature. In addition, there is a need for compositions that meet these requirements from an economic point of view and offer improved production processability.

SUMMARY OF THE INVENTION [0007] The present invention overcomes the above-mentioned shortcomings of the prior art by providing articles comprising an irradiated cross-linked polymer composition according to the object of claim 1 and a method for producing these articles according to the subject of claim 17. Applicants have found that the compositions of the invention are characterized by stiffness and toughness that are similar to the above-described compositions predominantly based on polypropylene, without the need for polypropylene as the dominant component. In addition, these compositions offer improved extrudability in terms of processing and production control.

[0008] The articles and compositions of the invention are suitable for use in applications that require a lower degree of heat resistance than is typically required for compositions containing predominantly

In polypropylene grade. More specifically, the articles and compositions of the invention are suitable for use at operating temperatures below the softening point of the medium density polyethylene component, the linear medium density polyethylene component or the high density polyethylene component or in which the mechanical deformation due to softening above this temperature is not is a limiting factor. Examples include radiation cross-linked, heat-shrinkable insulating sheaths for mechanical and corrosion protection of pipe joints at temperatures up to 110 ° C; irradiated, heat-shrinkable tubing products for mechanical protection, strain relief and electrical and electronic insulation of connections and terminations; and crosslinked by stress weaves, irradiation material for wires and cables, requirements are rigidity resistance and long-term temperature (up to 150 ° C).

electrical insulation for which it is important for dynamic load, high thermal stability

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0009] As mentioned above, the composition of the invention predominantly includes high density polyethylene (HDPE), medium density polyethylene (MDPE), or linear medium density polyethylene (LMDPE), referred to herein as " polyethylene". The use of the term "predominantly" when referring to the compositions of the invention is intended to mean at least 50 weight percent. Accordingly, the composition of the invention contains at least 50 weight percent polyethylene. Preferably, the polyethylene is included in the composition in an amount of from above 50 percent to about 90 percent, and more preferably from about 55 to 80 percent.

[0010] The terms HDPE and MDPE as used herein are defined in accordance with the standard of the American Research Association

EP 1 688 458 B1

Materials (American Society for Testing and Materials, ASTM) Standard D 1248. By definition, MDPE has a density in the range of 0.926 to 0.940 g / cm³, while HDPE has a density of at least 0.941 g / cm³. LMDPE density falls within the same density range as MDPE. By contrast, low-density polyethylene (LDPE) as defined in ASTM Standard D 1248 has a density of 0.910 to 0.925 g / cm³. Also, LDPE has a melting point of the crystalline substance no higher than 115 ° C.

[0011] The polyethylene used in the present invention preferably has a density of about 0.93 to 0.97 g / cm³, a melt flow index from about 0.1 to 10 dg / min, and a crystalline substance having a melting point of at least about 120 ° C. Polyethylene contains both ethylene homopolymers and copolymers of ethylene with higher alpha olefins, such as butene, hexene and octene, and has a predominantly linear molecular structure. Polyethylene may advantageously be produced using metallocene catalysts, also known as single-site, stereospecific or limited geometry catalysts, and have found molecular can also have a bimodal weight distribution in some cases, reporting that these materials provide the composition of the invention with the necessary crosslinking susceptibility additional cross-linking promoters.

[0012] The composition of the invention also contains polypropylene, which is selected from one or more members from the group consisting of polypropylene homopolymers and copolymers of propylene with an olefin other than propylene. Preferred polypropylene copolymers are copolymers of propylene and ethylene. In addition, the polypropylene component may advantageously be modified using reactive functional groups such as silanes, acrylic acids, methacrylic acids, acrylates, methacrylates, glycidyl methacrylates, and anhydrides. In the case where polypropylene is a copolymer, then it contains at least about 80 weight percent propylene.

[0013] The polypropylene component is present in the composition in an amount greater than about 5 percent and less than 50 percent by weight. Preferably, the polypropylene content of the composition is from about 5 to 40 weight percent, and more preferably from about 10 to 30 weight percent.

[0014] Polypropylene is preferably isotactic in nature, with a density of about 0.85 to 0.91 g / cm³ and a melt flow index of about 0.1 to 10 dg / minute. Preferably, the polypropylene has a polymer melt viscosity, as measured using a melt flow rate, which is similar to the viscosity of the HDPE component at the same temperature and under the same shear conditions required for processing the blend to ensure optimal blend uniformity. The melting point of the polypropylene crystalline substance usually ranges from about 160 to 170 ° C, and is typically about 165 ° C.

[0015] The composition of the invention optionally contains one or more ethylene propylene elastomers selected from the class of materials known as ethylene propylene copolymers or elastomers (EPM), more preferably from those known as ethylene propylene diene diene terpolymers or elastomers (EPDM), and most preferably of ethylene propylene diene terpolymers or elastomers polymerized using single site or metallocene catalysts (mEPDM), or mixtures thereof. The ethylene propylene elastomer component is preferably selected so as to have a polymer melt viscosity as close as possible to the polypropylene and polyethylene components at the same temperature and shear conditions required for processing the blend. The ethylene propylene elastomer component preferably has a density of about 0.85 to 0.92 g / cm³ and Mooney viscosity (ML 1 +4 at 125 ° C), which indicates a polymer melt viscosity of from about 5 to 50.

[0016] Preferably, the ethylene propylene elastomer contains from about 40 to 95% by weight ethylene, more preferably from about 70 to 95% by weight ethylene, even more preferably from about 75 to 95% by weight ethylene, and most preferably from about 85 to 95% by weight ethylene. Ethylene propylene diene terpolymers additionally contain from about 0.5 to 10% by weight of diene monomer, usually selected from 5-ethylidene-2-norbornene, dicyclopentadiene, or 1,4-hexadiene, and preferably 5-ethylidene-2-norbornene.

[0017] The most preferred mEPDM terpolymers are produced by the copolymerization reaction of propylene with ethylene and one or more diene monomers listed above, using highly stereospecific, single site, limited geometry, or so-called metallocene catalyst. These preferred mEPDM materials differ from EPDM materials produced using standard Ziegler-Natta coordination catalysts in that it is possible to more accurately control the amount and position of comonomers in the polymer structure to provide a more accurate molecular weight distribution and more regular molecular structure. resulting in, for example, greater crystallinity and better material properties. More important with respect to the present invention is that it is possible to regulate comonomer levels in order to achieve optimal crosslinking mEPDM materials by electron beam irradiation. In some cases, applicants have found that these materials provide the compositions of the invention with the necessary crosslinkability, without the need for additional crosslinking promoters.

[0018] The content of ethylene propylene elastomer in the composition of the invention is up to about 40%, more preferably up to about 30%, even more preferably from about to 20% by weight of the composition.

[0019] The composition may additionally comprise one or more optional ingredients selected from the group consisting of radiation sensitizers (also known as crosslinking promoters), homogenizing agents, pigments, antioxidant stabilizers, heat stabilizers, ultraviolet radiation stabilizers ( UV), mineral fillers, halogenated flame retardants, processing aids and the like.

[0020] The optional homogenizing agent may be selected from the group consisting of: any of the polyethylenes and polypropylenes described above; one or more members of the group consisting of ethylene propylene copolymers; ethylene propylene diene elastomers; crystalline propylene-ethylene elastomers; thermoplastic polyolefin elastomers; metallocene polyolefins; cyclic olefin copolymers; polyoctenamers; ethylene vinyl acetate copolymers, vinyl alcohol, and / or alkyl acrylates; polybutenes; hydrogenated and non-hydrogenated polybutadienes; butyl rubber; polyolefins modified using reactive functional groups selected from the group consisting of silanes, alcohols, amines, acrylic acids, methacrylic acids, acrylates, methacrylates, glycidyl methacrylates, and anhydrides; polyolefin ionomers; polyolefin nanocomposites; and block copolymers selected from the group consisting of styrenebutadiene, styrene-butadiene-styrene, styrene-ethylene / propylene and styrene-ethylene / butylene-styrene copolymers.

[0021] In any given composition of the invention, the homogenizing agent is different from the polyethylene, polypropylene components and the ethylene propylene elastomer component of this particular composition. Moreover, when the homogenizing agent comprises polyethylene, polypropylene or an ethylene propylene elastomer as defined above, the amount of homogenizing agent present in the composition is such that the total amounts of polyethylene, polypropylene and ethylene propylene elastomer in the composition are in the ranges described above for these ingredients.

[0022] The homogenizing agent is preferably added to the composition in an amount of up to about 25 percent by weight, more preferably up to about 15 percent, and even more preferably from about 5 to about 10 percent by weight of the composition so that the composition is still predominantly based on polyethylene. The function of the homogenizing agent is essentially to increase the miscibility of polypropylene and polyethylene components when they are mixed together, so that the blend exhibits homogeneous properties during use. The addition of a homogenizing agent is not necessary when the other components of the composition show satisfactory natural miscibility or effect.

[0023] The radiation sensitizer is preferably selected from the family of multifunctional monomers typically used as crosslinking promoters for polyolefin-based polymers. Preferred monomers include trimethylolpropane triacrylate, tetramethylolmethane tetraacrylate trimethacrylate, cyanuric acid triallyl ester and isocyanuric acid ester. The radiation sensitizer is preferably added in an amount of from about 0.25 to 2.5%, more preferably from 0.5 to 1.5% by weight of the blend. The function of the radiation sensitizer is to render the polymer composition more susceptible to cross-linking by means of electron beam irradiation (beta radiation) or gamma radiation, thus enabling a given level of crosslinking to be achieved using a lower dose and radiation energy than if the sensitizer had not been used. The addition of a crosslinking promoter is not necessary in case the composition is sufficiently susceptible to irradiation to obtain the required degree of crosslinking.

[0024] The antioxidant stabilizer may be selected from any suitable antioxidants or blend of antioxidants intended to prevent trimethylolpropane, ethylene glycol dimethacrylate, triallyl

EP 1 688 458 B1 degrades the molten product.

compositions during material processing and subsequent thermal final aging. Examples of suitable antioxidants and heat stabilizers include classes of chemicals known as bound phenols, bound amines, phosphites, bisphenols, benzimidazoles, phenylenediamines, and dihydroquinolines. These agents are preferably added in an amount of from about 0.1 to 5% by weight of the blend, depending on the type and amount of aging properties required and optional destabilizing ingredients in the composition, for example, halogenated flame retardants or mineral fillers. It should also be noted that stabilizers, if added in "free scavengers causing the reduction of these antioxidants and excessive amounts, may become radicals during irradiation", the irradiation efficiency in inducing the desired crosslinking reaction and the resulting degree of crosslinking obtainable for a given radiation dose.

[0025] The composition of the invention is produced by mixing polyethylene and polypropylene, together with any of the optional ingredients described above. Mixing of the ingredients may be carried out either as a separate step before processing the melt of the finished product, or it may be carried out simultaneously with the processing of the melt of the finished product.

[0026] In the case where mixing is carried out as a separate preliminary stage, the ingredients are preferably melt mixed using a machine specifically designed for this purpose, such as a single-screw or twin-screw mixer, kneader, or internal batch mixer used for continuous extrusion. The mixed composition can then be granulated and stored for subsequent processing of the molten material into the desired finished product.

[0027] Processing of the molten material of the composition may advantageously be carried out using techniques commonly used in industry, such as extrusion

EP 1 688 458 B1 or forming. Examples of extruded products include films, tubing and electrical insulation. In some preferred embodiments, the composition can be co-extruded or laminated with another material of similar or different composition to form a layered structure comprising individual but closely related layers, each layer having different performance characteristics. For example, an adhesive coated polymer film can be made by co-extrusion or laminating the composition with the adhesive. In other examples, the composition may be laminated with a less expensive or non-crosslinkable layer. Molded articles can be made using injection, compression or blow molding, and examples include electrical insulation products, such as end caps and interrupting protective sleeves.

[0028] After forming, the article is crosslinked by irradiation, preferably using an electron beam, gamma or UV radiation. Cross-linking means creating permanent covalent bonds between individual polymer chains that serve to bind the polymer chains together and prevent them from irreversibly separating during subsequent heating. It is this crosslinked structure that, while maintaining the elastomeric properties of the material, gives this material thermosetting and melting resistance, which in turn is a desirable property for producing heat-shrinkable products, as discussed below. Crosslinking also provides the product with high heat resistance, enabling it to maintain mechanical resistance to dynamic load and indivisibility at high operating temperatures.

[0029] Preferably, the article is irradiated by electron beam irradiation, at a dose of about 1 to 20 megarads, in an electron beam accelerator, for example "Dynamitron" produced by

Radiation Dynamics Inc. The desired dose depends on the desired

EP 1 688 458 B1 product properties. Too low a dose will lead to a product with a low degree of crosslinking, poor mechanical strength and a tendency to premature softening or melting at elevated temperatures. Too high a dose may cause degradation of the polypropylene component with unacceptable deterioration of mechanical properties. A preferred dose for the production of heat-shrinkable articles according to the invention has been found to be from 5 to 10 megarads. Higher doses may be more suitable for use as wire and cable insulation.

[0030] The radiation dose is sufficient to provide the product with a level of cross-linking, as measured using the gel fraction, representing from about 20 to 90 percent. Preferably, the gel fraction of the crosslinked article is from about 30 to 70 percent and more preferably from about 40 to 70 percent.

[0031] As mentioned above, the articles produced according to the invention can be made heat-shrinkable because they have the property that they do not melt when heated to a temperature close to or above the melting point of the crystalline substance of the component with the highest melting point, which is usually polypropylene . This is important because the crosslinked structure allows the product to be stretched with minimal force and without melting, and to retain its mechanical integrity when heated to this temperature. The article is cured in this stretched state by rapidly cooling to a temperature below the melting point of the crystalline substance, while maintaining the article in its stretched position, and the re-stiffened crystalline regions of the polymeric constituents of the material prevent the product from spontaneous returning to its original dimensions. The stretching of the product can be carried out using mechanical, pneumatic or hydraulic means. Cooling the product in its extended condition can be carried out using a cooling medium,

EP 1 688 458 B1 such as air, water or other heat exchange medium.

[0032] Subsequent reheating of the stretched product to a temperature above the melting point of the component with the highest melting point will cause the melting of the crystalline regions again and elastomeric recovery through the structure of its original unstretched dimensions. The crosslinked structure ensures the forcing to return to the initial state after deformation and again causes that the product does not melt and that it retains its mechanical indivisibility.

[0033] The invention is further illustrated using the following examples:

EXAMPLE 1 [0034] Isotactic polypropylene copolymer with a density of 0.90 g / cm³ and a melt flow index of 0.45 dg / minute (Profax 7823 from Basell Polyolefins), HDPE with a density of 0.947 g / cm³ and a melt flow index of 0.28 dg / minute and metallocene catalyzed ethylene propylene water terpolymer with a density of 0.908 g / cm³ and a flow rate of 1.0 dg / min (Nordel IP 4820P from DuPont Dow Elastomers) was melt mixed with an antioxidant masterbatch containing 15% bound phenolic and phosphite stabilizers in polyethylene (Irganox B225 from Ciba Specialty Chemicals) and a pigment masterbatch containing 25 % soot in polypropylene, in amounts given in Table 1 using a Buss Kneader kneading machine, piston screw, continuous extrusion molding mixer, at a temperature of about 180 ° C, and then the dispersed mix was passed through a granulator with a hot matrix surface and drying equipment.

[0035] The mixed granules produced in this way were passed through a 24: 1 L / D single-screw extruder

EP 1 688 458 B1 equipped with a matrix for extruding monolayer films, and pressed to form a film at a melting point of approximately 220 ° C. The film was fixed to the required dimensions of width, thickness and order by passing it through a cooled, 3-cylinder calendering system.

[0036] The extruded film was then crosslinked using an approximately 5 megarad radiation dose using the Radiation Dynamics "Dynamitron" electron beam accelerator, after which it was tested to determine the degree of crosslinking achieved and the mechanical properties, which are shown in Table 2.

[0037] Then, the crosslinked film was again heated to a temperature of approximately 150 ° C and stretched to obtain approximately 50% stretch, using a mechanical machine direction (MDO) machine. While in the stretched state, the film was quickly cooled by passing it between the water-cooled steel rollers to a temperature below the melting point of the crystalline substance of the composition to fix the film in a stretched state. Then, the film was laminated by extrusion with a layer of hot melt adhesive.

EXAMPLE 2 [0038] In this example, the process of example 1 was repeated, except that other relative amounts of HDPE, polypropylene and terpolymer were used as shown in table 1.

EXAMPLES 3, 4 and 5 [0039] In these examples, the effect of changing HDPE components was studied. In example 3, HDPE had a density of 0.960 g / cm³ and melt flow index equal

4.9 dg / minute. In Example 4, metallocene catalyzed HDPE with a density of 0.935 g / cm was used³ and

At a melt flow rate of 0.9 dg / minute. Example 5 uses the same HDPE as used in examples 1 and 2 above. The polypropylene to HDPE ratio was kept constant in all cases.

[0040] Examples were prepared by mixing the ingredients listed in Table 1 using a laboratory internal mixing system at a temperature of approximately 200 ° C. The mixed compositions were then pressed into platelets approximately 0.060 inches thick and then crosslinked as described in Example 1.

[0041] Next, the cross-linked plates were tested to determine the obtained degree of cross-linking and mechanical properties, which are shown in Table 2.

EXAMPLE 6 [0042] The cross-linked heat-shrinkable tubular product was prepared by extrusion-molding the composition described in Example 1, at a melting point of 220 ° C, obtaining a cylindrical cross-section, followed by irradiating the extruded tube using a radiation dose of about 5 megarads, heating cross-linked in this way pipes up to 190 ° C, stretching the pipe to double its original internal diameter by using internally compressed air, and finally spraying the stretched pipe with cold water to fix the pipe in its stretched state.

EXAMPLE 7 [0043] Crosslinked insulated wire was made by extrusion molding the composition described in example 5 at a melting point of 240 ° C on a 14 AWG wire, followed by

Irradiating said wire with a radiation dose of 12 megarads.

TABLE 1

<td colspan="6">compositions</td>

<td>Ingredient</td><td>Example 1</td><td>Example 2</td><td>Example 3</td><td>Example 4</td><td>Example 5</td>

<td>HDPE</td><td> 60</td><td> 80</td><td> 10</td><td> 20</td><td> 70</td>

<td>polypropylene (Profax 7823)</td><td> 30</td><td> 15</td><td> 30</td><td> 30</td><td> 30</td>

<td>EPDM (Nordel IP 4820P)</td><td> 10</td><td> 5</td><td></td><td></td><td></td>

<td>masterbatch antioxidant *</td><td> 12</td><td> 12</td><td> 7,5</td><td> 7,5</td><td> 7,5</td>

<td>masterbatch pigment **</td><td> 0,5</td><td> 0,5</td><td> 0,5</td><td> 0,5</td><td> 0,5</td>

<td colspan="6">* 15% masterbatch in polyethylene ** 25% masterbatch in polypropylene</td>

TABLE 2

<td>IN</td><td colspan="5">Process terms and properties</td>

<td>Property</td><td>Example 1</td><td>Example 2</td><td>Example 3</td><td>Example 4</td><td>Example 5</td>

<td>Dose (Mrad)</td><td> 5</td><td> 5</td><td> 8</td><td> 8</td><td> 8</td>

<td>Gel fraction (%)</td><td> 50</td><td> 70</td><td> 40</td><td> 45</td><td> 45</td>

<td>Resistance to stretching on hot at</td><td> 12</td><td> 13</td><td> 3,5</td><td> 5,5</td><td> 13,5</td>

EP 1 688 458 B1

<td>200 ° C and at 100% elongation (Psi)</td><td></td><td></td><td></td><td></td><td></td>

<td>Final extension on hot at 200 ° C (%)</td><td> &gt;450</td><td> 400</td><td> &gt;450</td><td> &gt;450</td><td> &gt;450</td>

<td>The final resistance to stretching in temp. 23 ° C (psi)</td><td> 4300</td><td> 3100</td><td> 2900</td><td> 2900</td><td> 3000</td>

<td>Final extension in temp. 23 ° C (%)</td><td> 600</td><td> 550</td><td> 13</td><td> 180</td><td> 10</td>

<td>Module elasticity in temp. 23 ° C (psi)</td><td> 63,000</td><td> 56, 000</td><td> 65,000</td><td> 55,000</td><td> 62,000</td>

[0044] Although the invention has been described in relation to certain preferred embodiments, it should be understood that this is not intended to limit its scope. It is intended that the invention rather includes all embodiments that fall within the scope of the claims below.

EP 1 688 458 B1